The main- and transfer-bus scheme adds
a transfer bus to the single-bus scheme. An extra bus-tie circuit
breaker is provided to tie the main and transfer buses together.

This scheme is arranged with all
circuits connected between a main (operating) bus and a transfer bus
(also referred to as an inspection bus). Some arrangements include a
bus tie breaker that is connected between both buses with no circuits
connected to it.

Since all circuits are connected to the
single, main bus, reliability of this system is not very high.
However, with the transfer bus available during maintenance,
de-energizing of the circuit can be avoided. Some systems are
operated with the transfer bus normally de-energized.

When a circuit breaker is removed from
service for maintenance, the bus-tie circuit breaker is used to keep
that circuit energized. Unless the protective relays are also
transferred, the bus-tie relaying must be capable of protecting
transmission lines or generation sources. This is considered rather
unsatisfactory because relaying selectivity is poor.

A satisfactory alternative consists of
connecting the line and bus relaying to current transformers located
on the lines rather than on the breakers. For this arrangement, line
and bus relaying need not be transferred when a circuit breaker is
taken out of service for maintenance, with the bus-tie breaker used
to keep the circuit energized.

When maintenance work is necessary, the
transfer bus is energized by either closing the tie breaker, or when
a tie breaker is not installed, closing the switches connected to the
transfer bus. With these switches closed, the breaker to be
maintained can be opened along with its isolation switches.

Then the breaker is taken out of
service. The circuit breaker remaining in service will now be connected to both circuits through the
transfer bus. This way, both circuits remain energized during
maintenance.

Since each circuit may have a different
circuit configuration, special relay settings may be used when
operating in this abnormal arrangement. When a bus tie breaker is
present, the bus tie breaker is the breaker used to replace the
breaker being maintained, and the other breaker is not connected to
the transfer bus.

A shortcoming of this scheme is that if
the main bus is taken out of service, even though the circuits can
remain energized through the transfer bus and its associated
switches, there would be no relay protection for the circuits.
Depending on the system arrangement, this concern can be minimized
through the use of circuit protection devices (reclosure or fuses) on
the lines outside the substation.

If the main bus is ever taken out of
service for maintenance, no circuit breakers remain to protect

any of the feeder circuits. Failure of
any breaker or failure of the main bus can cause complete loss

of service of the station.

Due to its relative complexity,
disconnect-switch operation with the main- and transfer-bus

scheme can lead to operator error and a
possible outage. Although this scheme is low in cost and

enjoys some popularity, it may not
provide as high a degree of reliability and flexibility as required.

This arrangement is slightly more
expensive than the single bus arrangement, but does provide more
flexibility during maintenance. Protection of this scheme is similar
to that of the single bus arrangement. The area required for a low
profile substation with a main and transfer bus scheme is also
greater than that of the single bus, due to the additional switches
and bus.